An apparatus for quantitatively determining a component of a gas flow is disclosed in U.S. Pat. No. 4,736,619. This apparatus is a breath alcohol measuring device wherein the breath is blown through a conduit into the ambient with the conduit having a restriction or throttle formed therein. A sample conduit branches off from the testing conduit upstream of the throttle and leads to an alcohol sensor. A pressure sensor is connected to the sample conduit for measuring the pressure at the throttle. The signal of the pressure sensor is integrated over time by a control unit during the time that the breath is blown into the testing tube. In this way, the air volume flowing through the testing tube into the ambient is determined. A sample-taking pump is actuated by the control unit only after so much air has flown that alveolar air is present in the testing conduit. The sample-taking pump pumps a specific quantity of air from the testing conduit through the sample conduit into the alcohol sensor. The alcohol content of the breath sample is then determined.
It is a disadvantage of this known apparatus that a sample-taking pump is necessary which, with its mechanical parts, causes the pump to be subject to failure and increases the cost of the apparatus. Furthermore, high requirements as to precision cannot be expected with a low-cost pump.